A transmission and reception module used in a radar and the like transmits a desired high-frequency wave via an antenna to a measurement object, and receives a high-frequency wave reflected by the measurement object via the antenna. Thus, a distance from the module to the measurement object can be measured.
A typical transmission and reception module includes an antenna serving as transmission and reception terminals of a high-frequency wave, and a transmission circuit and a reception circuit connected to the antenna via a circulator.
In particular, the radar is required to be capable of transmitting the high-frequency wave farther, and receiving a faint reflected wave reflected by the measurement object. Therefore, an amplifier is provided in each of the transmission circuit and the reception circuit.
However, the reception circuit in particular may receive a high-frequency wave reflected by the measurement object or an obstacle existing at a very short distance in some cases. In this case, the high-frequency wave having a power exceeding a breakdown power is input to the amplifier in the reception circuit. As a result, the amplifier of the reception circuit is broken down.
Furthermore, in the case where the measurement object exists at a very short distance, the high-frequency wave is multiply reflected between the antenna and the measurement object, which causes a transmission load VSWR of the transmission and reception module to considerably deteriorate. In this case, the transmission wave is reflected by the antenna and input to the reception circuit via the circulator. In this case also, the high-frequency signal having the power exceeding the breakdown power is input into the amplifier in the reception circuit. As a result, the amplifier of the reception circuit is broken down.
Therefore, a conventional transmission and reception module is provided with a limiter circuit in which a diode is used between the circulator and the amplifier of the reception circuit. This limiter circuit suppresses a power of the high-frequency wave input to the circuit to a desired power. That is, even when the transmission and reception module receives the high-power high-frequency wave, the above problem can be solved by suppressing the power of the high-frequency wave to the desired power with the limiter circuit before being input to the amplifier.
Limiter circuits are typically provided in multiple stages to process a higher power. More specifically, a high-power high-frequency diode limiter circuit capable of processing a high-power reception wave is provided in a first stage, and a low-power high-frequency diode limiter circuit is subsequently provided.
A diode which can process a high power is used in the high-power high-frequency diode limiter circuit. However, since a high-frequency wave loss is great in the above diode, there is a problem in performance that the high-frequency wave loss is great in the high-power high-frequency diode limiter circuit. Therefore, a noise figure is deteriorated in the reception circuit having the high-power high-frequency diode limiter circuit. In this case, a low-power high-frequency wave cannot be received, and a maximum detectable range is deteriorated.
When the measurement object positioned at a short distance is measured, there is a method to avoid using the high-power high-frequency diode limiter circuit by decreasing a pulse width of the transmission wave as an operational measure. However, this method cannot deal with the prevention of an electronic wave having a large pulse width transmitted from the measurement object.